In an assembly of a tire and a wheel rim, the tire bead portion is conventionally held to the beadbase by virtue of the pressing force of the inner pressure of the tire and the frictional force between the beadbase and the rubber of the bottom portion of the tire bead. Therefore, if the inner pressure of the tire is lowered, this holding force is reduced and, specifically, if the vehicle steers sharply with a rapidly lowered inner pressure caused by, for example, a puncture, the rim comes off the bead portion due to a force produced in the transverse direction generated by the turning operation. In particular, a rim provided with a well for mounting a tire involves the danger of bringing about a serious accident when the bead portion of the tire drops into the well and thereby completely comes off the rim. More specifically, the mechanism of the bead portion of a tire coming off the rim can be explained as follows.
The force in the transverse direction caused when a vehicle is cornering deviates the tread portion in the transverse direction relative to the wheel rim and produces deformation on the tread portion. This deformation is transmitted from the carcass to the bead portion of the tire. As a result, in the vicinity of the portion of the tire which comes into contact with the ground, a force is caused in the direction of the rotating axis of the tire (hereinunder referred to as "axial direction") and also a turning moment is produced around the circumference of the bead.
At this time, if the inner pressure of the tire is low, since these forces lift the heel of the bead portion, the frictional force between the bottom portion of the bead and the beadbase, which is the bead portion holding force, is reduced. As a result, the bead portion moves on the tapered bead base inwardly in the axial direction, and the tension of the bead core is also reduced with the movement of the bead portion. The bead portion holding force is therefore rapidly lost, whereby the bead portion drops into the well. To solve this problem, the use of a rim having no well, the use of a rim provided with a hump at the portion which comes into contact with the bead toe so as to prevent the bead portion from moving inwardly in the axial direction, an assembly of a tire and a rim having a hump provided in the beadbase and an annular groove provided at the bead bottom portion of the tire so as to be engaged with the hump (Japanese Patent Laid-Open No. 13802/1974), and an assembly of a tire and a rim having an annular groove provided axially inside the beadbase and a hump provided on the bead toe so as to be engaged with the annular groove (Japanese Patent Publication No. 15007/1982) have been proposed. Among these, the assembly disclosed in the Japanese Patent Publication No. 15007/1982 has an excellent effect as a bead holding mechanism.
Such an assembly, however, has a special configuration at the portion at which the rim comes into contact with the bead portion of the tire. It is therefore impossible to use a conventional tire with such a rim, and if a conventional tire is used by mistake, there is a problem in terms of safety.
Accordingly, it is an object of the present invention to provide a wheel rim and an assembly of a wheel rim and a tire which are capable of maintaining the bead portion holding mechanism of a tire when the inner pressure of the tire is lowered and which are also capable of utilizing a tire having a conventional structure.